Electrocatalytic oxygen reduction activity of AgCoCu oxides on reduced graphene oxide in alkaline media

Silver-based electrocatalysts as promising substitutes for platinum materials for cathodic oxygen electroreduction have been extensively researched. Electrocatalytic enhancement of the Ag nanoarchitectonics can be obtained via support structures and amalgamating Ag with one or two additional metals. The work presented here deals with a facile microwave-assisted synthesis to produce bimetallic Ag-Cu and Ag-Co (1:1) oxide nanoparticles (NPs) and trimetallic AgCuCo (0.6:1.5:1.5, 2:1:1, and 6:1:1) oxide NPs supported on a reduced graphene oxide (rGO) matrix. Morphology, composition, and functional groups were methodically analysed using various microscopic and spectroscopic techniques. The as-prepared electrocatalysts were employed as cathode substrates for the oxygen reduction reaction (ORR) in alkaline medium. Varying the Ag fraction in copper cobalt oxide has a significant influence on the ORR activity. At a ratio of 2:1:1, AgCuCo oxide NPs on rGO displayed the best values for onset potential, half-wave potential, and limiting current density (Jk) of 0.94 V vs RHE, 0.78 V, and 3.6 mA·cm−2, respectively, with an electrochemical active surface area of 66.92 m2·g−1 and a mass activity of 40.55 mA·mg−1. The optimum electrocatalyst shows considerable electrochemical stability over 10,000 cycles in 0.1 M KOH solution.


Materials characterization
An infrared spectrometer IR-Tracer 100 Schimadzu was used to record Fourier-transform infrared spectra (FTIR) of the prepared electrocatalysts. A powder X-ray diffractometer PANalytical X'pert3 was used to carry out powder X-ray diffraction (PXRD) measurements.
The morphology studies were carried out by using a scanning electron microscope (FEI QUANTA 200) with 20 kV accelerating voltage. Transmission electron microscopy (TEM) analyses of ACC-2 were carried out by using a JEOL JEM-2100 plus microscope (Japan). Xray photoelectron spectroscopy measurements on ACC-2 were carried out by using ULVAC-PHI, Inc; Model: PHI5000 Version Probe III. The water contact angles of ACC-2 and ACC-2 * (0.5-2 L) were measured using a KYOWA DMs-40 contact angle metre (sessile drop), halfangle technique fit, and FAMAS add-in software.

Electrochemical measurements
All electrochemical measurements were performed on an electrochemical workstation (760E, CH Instrument) using a standard three-electrode system, which comprises of a graphite rod as counter electrode, silver/silver chloride (Ag/AgCl in 3 M KCl solution) as reference electrode and catalyst-loaded glassy carbon (GC) as working electrode. The working electrode was prepared by drop casting the catalyst ink onto a surface of pre-cleaned rotating disk electrode (RDE, 3 mm in diameter) and a rotating ring-disk electrode (RRDE, 4 mm in diameter). The catalyst ink was prepared by following a procedure similar to our previous study [2]. By dispersing 4 mg of each catalyst in 1 mL of IPA solution containing 20 μL of 5 wt % Nafion, S4 followed by ultrasonication for 30 min. Thereafter, 4 L of catalyst ink was drop cast on RDE.
The catalyst loading on RDE-GC was maintained to be 226 µg·cm −2 during the electrochemical studies. The ORR performance of the catalysts was measured in O2-saturated 0.1 M KOH solution. The cyclic voltammetry (CV) curves were obtained at a scan rate of 20 mV·s −1 . The linear sweep voltammetry (LSV) was performed using RDE at a scan rate of 10 mV·s −1 with various rotation speeds (400-2500 rpm). All measured potentials are reported versus the reversible hydrogen electrode (RHE) [3]. The onset potential was defined as the potential required for generating a current density of 0.1 mA·cm −2 in LSV curves. The electron transfer number was calculated with the help of the Koutecky-Levich (K-L) equation: where is the measured current density, is the kinetic diffusion current density, d is the diffusion current density, is the slope, ω is the angular velocity (ω= 2πN, N is the rotation speed), is the number of transferred electrons, is the Faraday constant (96485 C·mol −1 ), 0 is the saturation concentration of O2 (1.2×10 −6 mol·cm −3 ), 02 is the diffusion coefficient of O2 (1.9×10 −5 cm 2 ·s −1 ), and ν is the kinematic viscosity (0.01 cm 2 ·s −1 ) [3].
The number of transferred electrons and the amount of generated hydrogen peroxide were investigated using RRDE measurements. The yield of hydrogen peroxide (H2O2) and the number of transferred electrons (n) were determined using the following equations: where is the disk current, is the ring current, and N is the current collection efficiency of the platinum ring (N = 0.38).
Moreover, the electrochemical active surface area (ECSA) was calculated via the double-layer capacitance using the following equation.

ECSA = CDL/Cs
where, CDL is double-layer capacitance and Cs represents the specific capacitance under alkaline conditions [4,5]. Finally, the stability of the catalyst was tested by electrochemical